I'm brand new to the industry (coming from dental sales business) so take what I'm saying with a grain of salt:
All I do all day every day is put my feet on the street and look for choice commercial accounts. With this company a proposal is fairly detailed and takes a lot of effort & time to complete so I make sure to identify decision makers and qualify them first before we agree to do a bid.
Based on my three plus months' experience (I know...a genuine wellspring of landscaping wisdom) I would venture to say that random proposals handed to a secretary or sent blindly to an "email@example.com" email address will just disappear into the corporate vacuum 99% of the time.
Businesses and office buildings with an onsite property manager are the best (easiest) targets and opportunities to pitch your services IMHO. Onsite managers are relatively easy to get face to face with and it's more difficult to blow somebody off in person as opposed to on the phone or via email.
So far, even the warm leads I've generated have required consistent follow up in order to convert them into jobs; that means you need to know exactly who is making the decision and how to reach them specifically.